Most passenger restraint belt systems use retractors of the emergency locking type in which the reel locks in response to a relatively high, abnormal acceleration or deceleration of the vehicle. It has been recognized that emergency locking retractors allow a rather considerable elongation of the belt, even though the reel is locked, because the several turns of the belt normally on the reel are initially loose and are pulled tight when the passenger is thrown forward and because the belt itself elongates.
It is, of course, desirable and sometimes necessary, especially in small automobiles, to limit belt elongation, and various ways of doing so have been proposed. Most of the proposals have been unsatisfactory either because they allow the belt to slip or present a risk of belt failure by cutting or digging into it. In many of the few devices which seem to lock the belt reasonably well, the lock is so effective that it is very difficult to release the belt, thus causing great inconvenience and sometimes real difficulty for a passenger who finds himself or herself strapped in the vehicle after a crash. If the belt does not release readily, there may, in fact, be great danger to the occupant in case of a fire or some other reason for rapid escape.